The invention relates to the deposition of coatings in channels. More particularly, the invention relates to the deposition of membrane coatings in honeycomb channels having small diameters. Even more particularly, the invention relates to a method and apparatus for depositing membrane coatings in honeycomb channels having small diameters.
A variety of coating methods, including dip-coating, slip casting, and spin coating have been used to deposit layers of materials—particularly membrane materials—on porous substrates or support structures. Such methods work well for coating flat surfaces, external surfaces of tubular structures, or internal surfaces of large diameter tubes.
However, the application of membrane coatings to multiple small sized (≦3 mm diameter) channels in a monolithic substrate or support presents challenges that are difficult to overcome using the previously mentioned methods. Such challenges include the introduction of viscous coating solutions into small channels, providing uniform coatings along the length and around the circumference of the channels, and controlling deposition conditions, such as residence time. In addition, the above methods are not effective in coating such channels using reactive processes, such as electroless plating, hydrothermal synthesis, and the like.
Whereas wash coating has been used to deposit catalyst layers on honeycomb substrates having small channels and high densities, this process is not applicable to the membrane coating process for several reasons. First, wash coating typically results in a coating layer having a number of cracks and a thickness that is greater than that desired for membrane films. Secondly, the length of the honeycomb structure that can be coated by wash coating is limited to about 6 inches (about 15 cm).
Presently available coating techniques are unable to provide uniform membrane coatings for small channels of honeycomb structures of longer length. Therefore, what is needed is a method of applying a uniform membrane coating to the channels of such honeycomb structures. What is also needed is an apparatus that enables the application of such membrane coatings to honeycomb structures having small channels.